The morphology of floral organs is an important factor for enhancing the decorative properties of ornamental plants. With regard to the morphogenesis of the floral organs, first, an inflorescence meristem differentiates into a flower primordium, and then a floral meristem contained in the flower primordium differentiates into four types of floral organs, namely, sepals, petals, stamens and pistils. Thereafter, a mature flower is formed as a complex organ including the floral organs.
A flower that is generally called “double-flowered” implies that the flower is a state in which more petals further line up at the inner side of petals, where stamens or pistils line up, and it seems as if the flower is constituted only of petals.
In general, the morphogenesis of the flower in higher plants may be explained by the ABC model. In the ABC model, the morphological change of the flower is explained by the transcriptional regulation of genes that belong to the MADS-box family (Non-Patent Literature 1). The MADS-box family is a gene family consisting of 30 or more kinds of genes, and the family is further divided into Class A, Class B and Class C. The genes that belong to the MADS-box family encode transcription factors containing a conserved region called the MADS-box. Genes such as APETALA1 (AP1) and APETALA (AP2) are classified as Class A genes; genes such as APETALA3 (AP3) and PISTILLATA (PI) are classified as Class B genes; and genes such as AGAMOUS (AG) are classified as Class C genes. It has been confirmed that variants of these genes present morphological changes of the flower.
Plants in which the morphology of petals or the number of petals has been modified have been hitherto obtained mainly by cross breeding of crossing varieties of plants, or by searching for mutants that occur in nature. However, in the case of cross breeding, a long time and skilled technicians are needed in order to produce plants having a desired morphology, and in the case of the search for mutants, the desired morphology is obtained only incidentally, there is a demand for a simple and easy method for reliably modifying the morphology of petals or the number of petals.
As the method for modifying the morphology of a plant by genetic engineering, methods of utilizing a functional peptide which converts an arbitrary transcription factor into a transcriptional repressor are known (Patent Literatures 1 to 7). These functional peptides are peptides excised from Class II ERF (Ethylene Responsive Element Binding Factor) protein, or from a plant zinc finger protein such as Arabidopsis thaliana SUPERMAN protein, and those peptides have simple structures. It has been reported in connection with Arabidopsis thaliana that the expression of AP3 gene or AG gene are inhibited by introducing a DNA that encodes a chimeric protein which is a fusion protein of AP3 protein or AG protein, and a functional peptide, into the plant, and thereby sterile males are produced or the number of petals is increased (Patent Literatures 8 and 9).
However, since Arabidopsis thaliana is an ornamental plant having only one type of AG gene, it can be contemplated that functions of the AG gene of Arabidopsis thaliana may be different from functions of AG genes of ornamental plants having plural AG genes. In fact, it has been reported that a chimeric protein obtained by fusing an AG protein of Arabidopsis thaliana and a functional peptide is not able to increase the number of petals in torenia (Non-Patent Literature 2). Furthermore, it is also known that the functions of various AG genes in ornamental plants having plural AG genes may be different depending on the type of the ornamental plant.
Cyclamen is popular as an ornamental plant for decorative purposes. However, there is no report on the AG gene of cyclamen, and nothing is known about the number and functions of the AG gene of cyclamen. 